1. Field of the Invention
The present invention relates to building routing tables at intermediate network nodes for routes within an autonomous system; and in particular to reducing the number of queries to discover alternative routes to a particular node.
2. Description of the Related Art
Networks of general purpose computer systems and specialized devices connected by external communication links are well known and widely used in commerce. The networks often include one or more network devices that facilitate the passage of information between the computer systems and devices. A network node is a network device or computer or specialized device connected by the communication links. An end node is a node that is configured to originate or terminate communications over the network. An intermediate network node facilitates the passage of data between end nodes.
Communications between nodes are typically effected by exchanging discrete packets of data. Information is exchanged within data packets according to one or more of many well known, new or still developing protocols. In this context, a protocol consists of a set of rules defining how the nodes interact with each other based on information sent over the communication links. Each packet typically comprises 1] header information associated with a particular protocol, and 2] payload information that follows the header information and contains information that may be processed independently of that particular protocol. The header includes information such as the source of the packet, its destination, the length of the payload, and other properties used by the protocol. Often, the data in the payload for the particular protocol includes a header and payload for a different protocol associated with a different layer of detail for information exchange.
The headers included in a packet traversing multiple heterogeneous networks, such as the Internet, typically include a physical (layer 1) header, a data-link (layer 2) header, an internetwork (layer 3) header and a transport (layer 4) header, as defined by the Open Systems Interconnection (OSI) Reference Model. The OSI Reference Model is generally described in more detail in Section 1.1 of the reference book entitled Interconnections Second Edition, by Radia Perlman, published September 1999, which is hereby incorporated by reference as though fully set forth herein.
The internetwork header provides information defining the source and destination address within the network. Notably, the path may span multiple physical links. The internetwork header may be formatted according to the Internet Protocol (IP), which specifies IP addresses of both a source and destination node at the end points of the logical path. Thus, the packet may “hop” from node to node along its logical path until it reaches the end node assigned to the destination IP address stored in the packet's internetwork header.
Routers and switches are network devices that determine which communication link or links to employ to support the progress of data packets through the network. A network node that determines which links to employ based on information in the internetwork header (layer 3) is called a router.
Some protocols pass protocol-related information among two or more network nodes in special control packets that are communicated separately and which include a payload of information used by the protocol itself rather than a payload of data to be communicated for another application. These control packets and the processes at network nodes that utilize the control packets are said to be in another dimension, a “control plane,” distinct from the “data plane” dimension that includes the data packets with payloads for other applications at the end nodes.
A routing protocol only exchanges control plane messages used for routing data packets sent in a different routed protocol (e.g., IP). A portion of a network under the network administration of a single authority, such as an enterprise or Internet service provider (ISP) is called a domain or an autonomous system (AS). To reduce the consumption of network resources and improve scalability, some routing protocols send only summarized routing information. Routing information for an AS is summarized at its boundaries with one or more other ASs at intermediate network nodes called border gateway nodes or border gateway (BG) routers. Routing information shared within the borders of one AS is exchanged using an interior gateway protocol (IGP). Example IGPs include the link state protocols such as the intermediate system to intermediate system (IS-IS) protocol and the open shortest path first (OSPF) protocol. Another IGP, developed by Cisco Systems of San Jose, Calif. for use in its routers, is the Enhanced Interior Gateway Routing Protocol (EIGRP). Some of the link-state protocols divide an autonomous system into multiple areas, flood all data for a unified routing database within and area, but send only summarized information between areas. Some IGPs, like EIGRP, send only summary information from each intermediate node in the autonomous system.
To determine the best route in IGPs that send only summary routing information, the summary routing information includes for each destination node, a measure of the cost (called a cost metric) to reach that destination node from the intermediate node (e.g., router) providing the summary information. Metrics of cost to traverse links in a network are well known in the art. A router receives such summary routing information from each neighboring router (neighbor) with which the router shares a direct communications link. The receiving router then determines the route (i.e., the best next hop, also called the best “path” herein) based on the cost metrics reported by all the neighbors and the costs to traverse the link to reach each of those neighbors.
In a current approach, when a router loses a route to a particular destination, and does not have a record in storage for an alternative path that is loop-free, the router sends a query to each neighbor, asking for the neighbor's routes and costs to the particular destination. A loop-free path from a particular router is one in which the next hop goes to a router that is not farther from the destination than the particular router itself. If the next hop goes to a farther router, subsequent hops are likely to come back to the particular router, thus forming a loop.
While suitable for many purposes, there is a disadvantage with this approach. If even one of the queried neighbors does not have a loop-free path, that neighbor queries all its other neighbors. The query can thus propagate throughout the autonomous system, or until no information is available at any intermediate node for the destination address that is the object of the original query. The number of hops the queries make is the scope of the query. Querying routers wait for all queried routers to respond before selecting a new route from among the reported paths. The larger the scope of the query, the longer the convergence time until the new route, if any, is selected. For autonomous systems with thousands of nodes, the query scope can easily reach dozens of hops and convergence times can easily exceed several seconds. A lost route for several seconds can lead to thousands of lost transactions, which can lead to losses of millions of dollars in some financial systems.
Based on the foregoing, there is a clear need for techniques to limit the scope of queries initiated when a router loses a route to a particular destination. In particular, there is a need to avoid querying at least some neighbors that do not have a loop-free path to a destination that is a subject of the query.